1. Field of the Invention
The present invention relates to an illumination panel which is capable of emitting a light, incident on an optical fiber from a light source in the form of a plane, from the side face of the optical fiber and illuminating a transparent display board, a liquid crystal or the like, from the back face thereof, with this light.
2. Description of the Related Art
An optical fiber having a core-sheath structure transmits a light passing through the interior of the core while wholely reflecting the light. It is known from, for example, Japanese Examined Patent Publication No. 46-42534 and Japanese Examined Utility Model Publication No. 53-24750 that parts of the sheath layer are broken at predetermined portions of this optical fiber to form flaws such as notches reaching the core, which act as light-leaking portions, and an illumination panel in which light leaking from these light-leaking portions is utilized for illumination is fabricated.
This conventional technique is disadvantageous in that light leaking from the light-leaking portion generally becomes darker as the light moves away from the light source, and the brightness of the display panel becomes uneven.
A technique for overcoming this disadvantage of the uneven display is proposed, for example, in Japanese Examined Patent Publication No. 61-24684 and Japanese Examined Patent Publication No. 61-24685. According to this conventional technique, the presence density of flaws forming the light-leaking portion is controlled along the longitudinal direction of the optical fiber so that the flaw density can be expressed by a certain function, whereby a good brightness uniformity is obtained.
More specifically, if the density n of flaws formed at each position r of the optical fiber is controlled so that the flaw density is expressed by the following formula: ##EQU1## wherein r.sub.0 represents the illumination-starting position of the optical fiber and C represents a constant,
a good brightness uniformity can be obtained in the longitudinal direction. The outline of a formation of a light-leaking portion of the optical fiber and the relationship between the above-mentioned flaw density n and the distance from the end on the side of the light source are shown in FIG. 7.
As a practically effective means for forming light-leaking portions in an optical fiber sheet, there is generally adopted the hot stamping method in which an emery paper is placed on the optical fiber sheet, and a heating plate is pressed onto the emery paper and fine particles fixed to the surface of the emery paper are protruded into the sheath layer of the optical fiber by heat and pressure to break the sheath layer.
Where the light-leaking portion is formed by this hot stamping method, the change of the flaw density in the light-leaking portions along the longitudinal direction is realized, for example, by spreading a cushion sheet formed of a rubber below the optical fiber sheet and changing the thickness of the cushion sheet according to need, to partially control the compressive force applied when the heating plate is pressed to the optical fiber sheet.
Nevertheless, although the presence density of flaws formed in the light-leaking portions can be changed by the hot stamping method, the thickness of the cushion sheet must be changed substantially in compliance with the above-mentioned function of the conventional technique, to control the flaw density. Practically, the necessary control range of the thickness is within several hundred microns, and it is very difficult and industrially almost impossible to control the thickness of the cushion sheet at such a level, in compliance with the function.
Furthermore, even if this control of the thickness of the cushion sheet is possible, when a light is incident on the optical fiber sheet having thus-formed light-leaking portions from a light source, the spread of the light (light quantity distribution, outgoing angle distribution and the like) differs according to the kind of light source, and therefore, by controlling only the flaw density it is not easy to control the probability of the light leakage and the light exposure quantity, and bring the residual light quantity close to zero.
If a formation of flaws on an optical fiber sheet, fabricated by arranging optical fibers in the form of a sheet, by the hot stamping method is attempted, it is substantially impossible to form light-leaking portions having a pattern of the conventional technique as shown in FIG. 7. In FIG. 7, the light-leaking portion is expressed as being formed along the entire width of the optical fiber in the plane view and is seen to be present along almost 1/2 of the circumference of the optical fiber. In practice, however, where a plurality of optical fibers are arranged in the form of a sheet, it is impossible to form a light-leaking portion along 1/2 of the circumference, as seen from FIG. 4, and the region of the light-leaking portion must be reduced to a fraction of 1/2 of the circumference, with the result that the probability of light leakage is greatly reduced, and thus by controlling the flaw density only as taught by the conventional technique, the probability of the light leakage and the leaking light quantity cannot be sufficiently controlled and it is impossible to realize a high-performance illumination panel in which the residual light quantity is small and the light utilization ratio is high.
According to the sand blasting method, as another conventional technique of forming light-leaking portions, fine particles such as sand grains or glass beads are jetted from a nozzle against an objective material, and light-leaking portions can be formed substantially along 1/2 of the circumference, but because of characteristics of the sand blasting method, it is difficult to control the flaw density in compliance with the function of n=(C.e.sup.-(r-r.sbsp.0.sup.)).sup.-1 only by jetting fine particles, and the flaw density cannot be made substantially constant over the entire region.
Furthermore, according to a method comprising pressing a projecting member such as a needle into an optical fiber, it is industrially substantially impossible to realize an illumination panel having light-leaking portions controlled as taught in the conventional technique.